Carboniferous bimodal volcanic rocks in the Eastern Tianshan, NW China: Evidence for arc rifting

Abstract

Detailed geochronological, geochemical, and Sr–Nd isotopic data are presented for late Paleozoic volcanic rocks in the Dashitou area from the Eastern Tianshan, NW China, aiming to constrain their petrogenesis and tectonic implications. The Dashitou volcanic rocks show a bimodal distribution in composition, with dominant rhyolite and subordinate basalt. LA–ICPMS zircon U–Pb age indicates that the volcanic rocks were erupted at 330±3Ma. The basaltic rocks are medium-K calc-alkaline, enriched in light rare earth element (LREE) and large ion lithophile element (LILE; Ba and U), and depleted in high field strength element (HFSE; Nb, Ta, and Ti). These features, together with their depleted isotopic signature (initial 87Sr/86Sr=0.7035 to 0.7039, εNd(t)=6.8 to 7.1), suggest that they were likely derived from a depleted mantle source metasomatized by subduction-related fluids. The rhyolitic rocks are high-K calc-alkaline and exhibit characteristics of A2-type granite, with typical enrichment in alkalis, Zr, and Nb and depletion in Sr, P, and Ti. They are characterized by positive εNd(t) values (6.2–6.5) with young Nd model ages (0.57–0.58 Ga). Results of MELTS modeling imply that they were unlikely produced by fractional crystallization of coeval basaltic rocks. Instead, they were probably generated by partial melting of juvenile basaltic crust as a result of magma underplating. The geochemical and Sr–Nd isotopic characteristics, coupled with regional geology, indicate that the formation of the Dashitou bimodal volcanic rocks involves an extensional regime associated with a subduction-related environment. The rifting of the Dananhu–Harlik arc in response to the retreat of the subducting Northern Tianshan oceanic lithosphere may account for the Dashitou bimodal volcanic rocks.